This invention relates to through air drying of cellulosic webs and more particularly to through air drying with noncircular drying arrangements.
Through air drying is well known in the papermaking art. Through air drying is one means of removing water from an embryonic web comprising cellulose fibers. The following discussion is directed to tissue paper, although the invention is not so limited. The invention may be applied to any generally planar sheet material where it is desired to move air, or other compressible fluids, therethrough.
Through air drying has the advantage that structured paper, i.e., paper having regions which vary in density and having improved softness, caliper and absorbency, can be provided. Through air drying may be performed with one or more cylindrical through drying cylinders as illustrated by U.S. Pat. No. 4,953,297 issued Sep. 4, 1990 to Eskelinen et al; U.S. Pat. No. 5,411,636 issued May 21, 1993 to Hermans et al.; U.S. Pat. No. 5,601,871 issued Feb. 11, 1997 to Krzysik et al.; and European Pat. App. 0677612A2 published October 1995 in the names of Wendt et al. Capillary dewatering, using a cylindrical drying apparatus, is taught in commonly assigned U.S. Pat No. 4,556,450 issued Dec. 3, 1985 to Chuang et al., disclosure of which is incorporated herein by reference. Another generally cylindrical capillary dewatering apparatus is illustrated in U.S. Pat. No. 5,598,643 issued Feb. 4, 1997, U.S. Pat. No. 5,699,626 issued Dec. 23, 1997 and U.S. Pat. No. 5,701,682 issued Dec. 30, 1997, both to Chuang et al.
Another means of removing water from tissue paper is conventional felt drying. Conventional felt drying may also produce structured paper as illustrated by commonly assigned U.S. Pat. Nos. 5,549,790, issued Aug. 27, 1996 to Phan; U.S. Pat. No. 5,556,509, issued Sep. 17, 1996 to Trokhan et al.; U.S. Pat. No. 5,580,423, issued Dec. 3, 1996 to Ampulski et al.; U.S. Pat. No. 5,609,725, issued Mar. 11, 1997 to Phan; U.S. Pat. No. 5,629,052 issued May 13, 1997 to Trokhan et al.; U.S. Pat. No. 5,637,194, issued Jun. 10, 1997 to Ampulski et al.; U.S. Pat. No. 5,674,663, issued Oct. 7, 1997 to McFarland et al.; U.S. Pat. No. 5,693,187 issued Dec. 2, 1997 to Ampulski et al.; U.S. Pat. No. 5,709,775 issued Jan. 20, 1998 to Trokhan et al.; U.S. Pat. No. 5,776,307 issued Jul. 7, 1998 to Ampulski et al.; U.S. Pat. No. 5,795,440 issued Aug. 18, 1998 to Ampulski et al.; U.S. Pat. No. 5,814,190 issued Sep. 29, 1998 to Phan; U.S. Pat. No. 5,817,377 issued Oct. 6, 1998 to Trokhan et al.; U.S. Pat. No. 5,846,379 issued Dec. 8, 1998 to Ampulski et al.; U.S. Pat. No. 5,855,739 issued Jan. 5, 1999 to Ampulski et al.; U.S. Pat. No. 5,861,082 issued Jan. 19, 1999 to Ampulski et al., U.S. Pat. No. 5,871,887 issued Feb. 16, 1999 to Trokhan et al.; U.S. Pat. No. 5,897,745 issued Apr. 27, 1999 to Ampulski, et al.; U.S. Pat. No. 5,944,811 issued May 18, 1999 to Ampulski et al.; and U.S. Pat. No. 6,051,105, issued Apr. 18, 2000 to Ampulski, incorporated herein by reference.
Improvements to the through air drying process have occurred through utilizing micropore drying. Micropore drying occurs when a micropore drying medium is disposed in the flow path of the through air drying apparatus. The micropore drying medium has flow channels smaller than the interstices between the fibers of the embryonic web. Using micropore drying, the flow is controlled by the micropore drying medium, rather than by the web. Thus, differences in size of the web intersticesxe2x80x94e.g., as occur with regions of differing density between various regions of the webxe2x80x94do not affect air flow through the web. Micropore drying thus provides the advantage of more uniform drying of structured paper. Examples of micropore drying are illustrated in commonly assigned U.S. Pat. Nos. 5,274,930; 5,437,107; 5,539,996; 5,581,906; 5,584,126; 5,584,128; and 5,625,961, the disclosures of which are incorporated herein by reference.
The micropore drying apparatus according to the present invention may comprise a single zone. This zone is maintained at a differential pressure (either subatmospheric or superatmospheric) which causes breakthrough in the interstitial flow channels, or pores, of the micropore drying medium. Alternatively, the micropore drying apparatus may comprise two or more zones. The first zone may be maintained at a differential pressure which does not cause breakthrough in the pores of the micropore drying medium. The second zone, as well as any subsequent zones, may be maintained at a differential pressure such that breakthrough does occur.
However, installing a through air drying roll or a micropore drying roll in an existing plant may be infeasible. Papermaking machinery is large. Typical rolls are several feet in diameter, heavy and expensive. Sufficient space for a cylindrical drying apparatus, such as a micropore drying roll, may not exist. Moreover, often, one cannot economically justify retrofitting a cylindrical roll into the papermaking machinexe2x80x94no matter how desirable the end result may be. The cost of the apparatus may not pay out over time.
Furthermore, if such a generally cylindrical apparatus is economically feasible today, it may not be economically justifiable tomorrow. As bottlenecks disappear due to improvements elsewhere in the papermaking machine, the papermaking process becomes faster. As the papermaking process becomes faster, the residence time on each component decreases. But, it is necessary to provide a sufficient residence time on each component of the papermaking machinexe2x80x94without increasing the space requirements of that component.
A cylindrical apparatus may not have sufficient diameter to provide the necessary residence time to achieve drying at commercially viable speeds.
This invention provides the benefit that increased residence time is obtained without increasing the horizontal space requirements of the papermaking machine. This invention further provides the benefit of being usable with a micropore drying apparatus.
The invention comprises an apparatus for drying a web thereon. The drying apparatus has a machine direction and a profile orthogonal thereto. The drying apparatus comprises a micropore drying medium having a noncircular profile.
The drying apparatus may comprise a movable micropore drying medium. In such an execution, the micropore drying medium may comprise an endless belt which carries a web to be dried thereon. Optionally, a through air drying belt may move in tandem with the micropore drying medium and the web to be dried.
In yet another execution, the micropore drying medium may be stationary. In such an execution, the web is carried by a separate support member such as a through air drying belt.